Image Name as Hover Text — BUG?

Routinely, images in several WordPress websites are displaying the image title (file name) as hover text, even though we did not choose this option–ever! Is this a bug or feature? I would like to disable this feature, and have troubleshooted, but cannot find the source of the “alt text.” Can you help?

http://www.californialions.com
http://www.fridtjofnansenlodge.com
http://www.rikudim.net

Thank you!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!